1. Field of the Invention
The present invention relates generally to cellular communications systems. More particularly, the present invention relates to a novel and improved technique for handing off communication between base stations of different cellular systems.
2. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is but one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), and frequency division multiple access (FDMA) are known, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference.
In the just mentioned patent, a multiple access technique is disclosed in which a large number of mobile telephone system users, each having a transceiver (also known as a remote unit), communicate through satellite repeaters or terrestrial base stations (also known as base stations or cell-sites) using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques thus permitting an increase in system user capacity.
The conventional FM cellular telephone systems used within the United States is commonly called the Advanced Mobile Phone Service (AMPS), and is detailed in Electronic Industry Association standard EIA/TIA-553 "Mobile Station--Land Station Compatibility Specification". In such a conventional FM cellular telephone system, the available frequency band is divided into channels typically 30 Kilo Hertz (kHz) in bandwidth. The system service area is divided geographically into base station coverage areas which may vary in size. The available frequency channels are divided into sets. The frequency sets are assigned to the coverage areas in such a way as to minimize the possibility of co-channel interference. For example, consider a system in which there are seven frequency sets and the coverage areas are equally sized hexagons. The frequency set used in one coverage area is not used in the six nearest neighboring coverage areas.
In conventional cellular systems, a handoff scheme is used to allow a communication connection to continue when a remote unit crosses the boundary between coverage areas of two different base stations. In the AMPS system, the handoff from one base station to another is initiated when the receiver in the active base station handling the call notices that the received signal strength from the remote unit has fallen below a predetermined threshold value. A low signal strength indication implies that the remote unit must be near the coverage area boundary of the base station. When the signal level falls below the predetermined threshold value, the active base station asks the system controller to determine whether a neighboring base station receives the remote unit signal with better signal strength than the current base station.
The system controller in response to the active base station inquiry sends messages to the neighboring base stations with a handoff request. Each of the base stations neighboring the active base station employs a special scanning receiver which looks for the signal from the remote unit on the channel at which it is operating. Should one of the neighboring base stations report an adequate signal level to the system controller, a handoff is attempted to that neighboring base station which is now labeled the target base station. Handoff is then initiated by selecting an idle channel from the channel set used in the target base station. A control message is sent to the remote unit commanding it to switch from the current channel to the new channel supported by the target base station. At the same time, the system controller switches the call connection from the active base station to the target base station. This process is referred to as hard handoff. The term hard is used to characterize the `break-before-make` characteristic of the handoff.
In the conventional system a call connection is dropped (i.e. discontinued) if the handoff to the target base station is unsuccessful. There are many reasons that a hard handoff failure may occur. Handoff can fail if there is no idle channel available in the target base station. Handoff can also fail if one of the neighboring base stations reports receiving a signal from the remote unit, when in fact the base station actually is receiving a different remote unit signal using the same channel to communicate with a distant base station. This reporting error results in the transfer of the call connection to a wrong base station, typically one in which signal strength from the actual remote unit is insufficient to maintain communications. Furthermore, should the remote unit fail to receive the command to switch channels, the handoff fails. Actual operating experience indicates that handoff failures occur frequently which significantly lowers the reliability of the system.
Another common problem in the conventional AMPS telephone system occurs when the remote unit remains for an extended period of time near the border between two coverage areas. In this situation the signal level tends to fluctuate with respect to each base station as the remote unit changes position or as other reflective or attenuative objects within the coverage area change position. The signal level fluctuations can result in a "ping-ponging" situation in which repeated requests are made to handoff the call back and forth between the two base stations. Such additional unnecessary handoffs increase the probability that the call is inadvertently discontinued. In addition, repeated handoffs even if successful can adversely effect signal quality.
In U.S. Pat. No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM", issued Mar. 31, 1992, which is assigned to the assignee of the present invention, a method and system are disclosed for providing communication with the remote unit through more than one base station during the handoff of a CDMA call. Using this type of handoff communication within the cellular system is uninterrupted by the handoff from the active base station to the target base station. This type of handoff may be considered as a "soft" handoff in that concurrent communications is established with the target base station which becomes a second active base station before communication with the first active base station is terminated.
An improved soft handoff technique is disclosed within U.S. Pat. No. 5,267,261, entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM", issued Nov. 30, 1993, hereinafter referred to as the '261 patent, which is also assigned to the assignee of the present invention. In the system of the '261 patent, the soft handoff process is controlled based on measurements at the remote unit of the strength of "pilot" signals transmitted by each base station within the system. These pilot strength measurements assist the soft handoff process by facilitating identification of viable base station handoff candidates.
Remote unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the remote unit. The Active Set is the set of base stations through which active communication is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a signal strength of sufficient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength at a sufficient signal level to establish communication. The Remaining Set is a set of base station which are not members of the Active, Candidate, or Neighbor set.
When communications are initially established, a remote unit communicates through a first base station and the Active Set contains only the first base station. The remote unit monitors the pilot signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, T.sub.-- ADD, the base station is added to the Candidate Set and removed from the Neighbor Set at the remote unit. The remote unit communicates a message to the first base station identifying the new base station. The system controller decides whether to establish communication between the new base station and the remote unit. Should the system controller decide to do so, the system controller sends a message to the new base station with identifying information about the remote unit and a command to establish communications therewith. A message is also transmitted to the remote unit through the first base station. The message identifies a new Active Set that includes the first and the new base stations. The remote unit searches for the new base station transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations.
When the remote unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold, T.sub.-- DROP, for a predetermined period of time, T.sub.-- TDROP, the remote unit generates and transmits a message to report the event. The system controller receives this message through at least one of the base stations with which the remote unit is communicating. The system controller may decide to terminate communications through the base station having a weak pilot signal strength.
The system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The new Active Set does not contain the base station through which communication is to be terminated. The base stations through which communication is established send a message to the remote unit. The system controller also communicates information to the base station to terminate communications with the remote unit. The remote unit communications are thus routed only through base stations identified in the new Active Set.
The message sent from the remote unit to the base station comprising the pilot signal information of the members of the Candidate and Active Sets is referred as a Pilot Strength Measurement Message (PSMM). A PSMM is sent by the remote unit either in response to a request from the base station or because the signal strength of a base station of the Neighbor Set has exceeded a threshold, or the signal strength of a base station in the Candidate Set has exceeded the strength of one of the base stations of the Active Set by a predetermined amount, or the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold, T.sub.-- DROP, for a predetermined period of time, T.sub.-- TDROP.
Four parameters control the soft handoff process. First, the pilot detection threshold, T.sub.-- ADD, specifies the level which the pilot signal strength of a base station which is a member of the Neighbor Set must exceed to become classified as a member of the Candidate Set. The pilot drop threshold, T.sub.-- DROP, specifies the level below which the pilot signal strength of a base station which is a member of the Active or Candidate Set must fall in order to trigger a timer. The duration of the triggered timer is specified by T.sub.-- TDROP. After the time specified by T.sub.-- TDROP has passed, if the pilot signal strength is still below the T.sub.-- DROP level, the remote unit initiates removal of the corresponding base station from the set to which it currently belongs. The Active Set versus Candidate Set comparison threshold, T.sub.-- COMP, sets the amount by which the pilot signal strength of a member of the Candidate Set must exceed the pilot signal strength of a member of the Active Set to trigger a PSMM. Each of these four parameters is stored in the remote unit. Each of these four parameters can be reprogrammed to a new value by a message sent from a base station.
Although the foregoing techniques are well-suited for call transfers between base stations in the same cellular system which are controlled by the same system controller, a more difficult situation is presented by movement of the remote unit into a coverage area serviced by a base station controlled by a different controller. When a handoff must be made between base station controlled by different controllers, the handoff is called an intersystem handoff. One complicating factor in such intersystem handoffs is that typically there is no direct link between the base stations of the first system and the system controller of the second system and vice versa. The two systems are thereby precluded from performing simultaneous remote unit communication through more than one base station during the handoff process. Even when the existence of an intersystem link between the two systems is available to facilitate intersystem soft handoff, often dissimilar characteristics of the two systems further complicate the soft handoff process.
When resources are not available to conduct intersystem soft handoffs, the execution of a "hard" handoff of a call connection from one system to another becomes critical if uninterrupted service is to be maintained. The intersystem handoff must be executed at a time and location likely to result in successful transfer of the call connection between systems. It follows that the handoff should be attempted only when, for example:
(i) an idle channel is available in the target base station,
(ii) the remote unit is within range of the target base station and the active base station, and
(iii) the remote unit is in a position at which it is assured of receiving the command to switch channels.
Ideally, each such intersystem hard handoff should be conducted in a manner which minimizes the potential for "ping-ponging" handoff requests between the base stations of different systems.
These and other shortcomings of existing intersystem handoff techniques impair the quality of cellular communications, and may be expected to further degrade performance as competing cellular systems continue to proliferate. Accordingly, there is a resulting need for an intersystem handoff technique capable of reliably executing the handoff of a call between the base stations of different systems.